sha1_file: add the ability to parse objects in "pack file format"

sha1_file: add the ability to parse objects in "pack file format"

The pack-file format is slightly different from the traditional git
object format, in that it has a much denser binary header encoding.
The traditional format uses an ASCII string with type and length
information, which is somewhat wasteful.

A new object format starts with uncompressed binary header
followed by compressed payload -- this will allow us later to
copy the payload straight to packfiles.

Obviously they cannot be read by older versions of git, so for
now new object files are created with the traditional format.
core.legacyheaders configuration item, when set to false makes
the code write in new format for people to experiment with.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>